This invention relates to optical communications, and in particular to processing of arbitrary optical signals for inputting to all-optical clock recovery systems.
Optical fiber networks, such as SONET, are in widespread use due to their ability to support high bandwidth connections. The bandwidth of optical fibers runs into gigabits and even terabits. Optical links can thus carry hundreds of thousands of communications channels multiplexed together.
As in all modem data networks, information is encoded as digital digits of xe2x80x9conesxe2x80x9d or xe2x80x9czeros.xe2x80x9d There are various formats used to encode these digital bits as optical signals, depending on what type of pulse in the optical signal represents a xe2x80x9conexe2x80x9d digit. Two of the most common formats are Return to Zero (RZ) and Non Return to Zero (NRZ). In the former, the second half of each bit is a zero, represented by no amplitude in the optical signal. Thus, each bit, no matter what its value, has the zero amplitude level for its second half (in the time domain, obviously). The latter format, NRZ, does not return to a zero amplitude level each bit. Thus a sequence of 100 xe2x80x9conexe2x80x9d bits would be represented by holding the optical signal amplitude high for the time span of 100 bits. Only when the bit is actually a zero does the signal amplitude go low.
For obvious reasons, the non-return to zero format uses bandwidth more efficiently. The non-return to zero format is much more popular, and modem data networks tend to use a non-return to zero, or NRZ format.
The problem with NRZ format is that unless you know the inherent clock, it is very difficult to determine what the clock rate is of a given NRZ signal. This is because if you have a string of high, or xe2x80x9conexe2x80x9d bits, the NRZ signal simply stays high; if you have a sequence of low bits, it simply stays low, there being no regular transitions. Thus, as an example, in NRZ format, three high bits followed by three low bits followed by three high bits followed by another three low bits could be read as either 111000111000, or as 1010, with a clock speed one third as fast as the first alternative.
Whatever method is devised to process an incoming signal so that it can be submitted to clock recovery analysis in the optical domain, that method must be able to preprocess the incoming optical signals regardless of not only what format (RZ or NRZ) they happen to be in, but it also must be insensitive to the bit rate they happen to be in. As is known, there are varying nominal bit rates supported in a network, such as, for example, 10 GHZ, 20 GHZ, 30 GHZ and 40 GHZ, as well as various modifications to same resulting in various actual bit rates. This is due to error correcting codes and similar format specific modifications to the bit rate which use extra, non-data, bits for various management functions.
As optical networks become increasingly transparent, there is a need to recover the line rate in the form of a clock signal without resulting to any optical to electrical and back to optical conversion of the signal as is commonly done in the art. As networks tend towards optical transparency, the nodal devices in the optical network must work with all supported line rates, independent of their format. One of the fundamental functions of these devices will be the capability to extract the clock from the signal in the optical domain. This depends on the signal""s RF spectrum.
The RF spectrum of an RZ signal reveals a strong spectral component at the line rate. Consequently, the incoming RZ signal can be used directly to extract the clock signal. All that needs to be done with the incoming RZ signal is to amplify it. In the case of the NRZ signal format, the RF spectrum reveals no spectral component at the line rate, as described above. Thus, the RF spectrum of an ideal NRZ signal looks like a sinc function with the first zero at the line rate. Therefore, the fundamental problem of all optical clock recovery from NRZ signals is the generation in the signal of an RF spectral component at the line rate. Correlatively, the fundamental problem of all optical clock recovery from an arbitrary signal is the simple amplification of an RZ signal, and the conversion of an NRZ signal to one with a significant spectral component at the line rate.
The output of such processing can then be fed to clock recovery systems, which generally require an input signal with significant spectral component at the clock rate.
What is needed, therefore, to facilitate the next generation of transparent, all optical data networks, is a preprocessor which can take a given arbitrary optical signal, pass the signal with amplification if it is in RZ or other format with a large spectral component at the line rate, and take an incoming signal which does not have the large spectral component at the line rate and process it so that it does. All the processing that is to be done on the incoming arbitrary signal must only occur in the optical domain, so that optical-electrical-optical conversion, which is both costly and adds complexity, is not required.
A method and circuit are disclosed for the preprocessing of an arbitrary optical data signal for later use by a clock recovery system.
The method passes RZ signals with amplification, and converts incoming NRZ signals to Pseudo Return to Zero, or PRZ signals. PRZ signals have a strong spectral component at the line rate, and can be used by all optical clock recovery systems to lock onto that rate, thus generating an optical clock signal.
In a preferred embodiment, the method is implemented via a Semiconductor Optical Amplifierxe2x80x94Asymmetric Mach-Zehnder Interferometer, or SOA-AMZI, preprocessor, which, by controlling the SOAs in each arm, passes RZ signals with amplification, and converts NRZ signals to PRZ type signal, which has the requisite significant spectral component at the inherent clock rate.
The method is bit rate independent.
In an alternative embodiment, the preprocessor is implemented using a multimode coupler interferometer (MMCI). An exemplary SOA device for use in implementing the various circuits is also disclosed. Using such a SOA device, the entire preprocessor can be integrated on a single chip, thus facilitating all-optical integrated circuit network nodal processing functionalities.